The present invention relates to imaging techniques and apparatus for performing such techniques.
In magnetic resonance imaging (“MRI”), a strong, uniform magnetic field is applied to the region of the patient to be imaged. Radio frequency (“RF”) energy is applied to this region of the patient by a transmitter and antenna. The RF energy excites atomic nuclei within the patient's tissues. The excited nuclei spin at a rate dependent upon the magnetic field. As they spin, they emit faint RF signals, referred to herein as magnetic resonance signals. By applying small magnetic field gradients so that the magnitude of the magnetic field varies with location within the patient's body, the magnetic resonance phenomenon can be limited to only a particular region or “slice” of the patient's body, so that all of the magnetic resonance signals come from that slice. Moreover, by applying additional magnetic field gradients, the frequency and phase of the magnetic resonance signals from different locations within the slice can be made to vary in a predictable manner depending upon the position within the slice. Stated another way, the magnetic resonance signals are “spatially encoded,” so that it is possible to distinguish between signals from different parts of a slice.
If this process is repeated numerous times to elicit signals using different gradients, it is possible to derive a set of information which indicates one or more characteristics of magnetic resonance signals from particular locations within the patient's body. Such a set of information is referred herein to as an image data set. Because the characteristics of the magnetic resonance signals vary with the concentration of different chemical substances and other chemical characteristics of the tissues, different tissues provide different magnetic resonance signal characteristics. When a magnetic resonance signal image data set is displayed in a visual format, such as on a computer screen or printed image, the information forms a picture of the structures within the patient's body, with different tissues having different intensities or colors.
Typically, a magnetic resonance image data set is stored as a set of individual data elements. The data in each element represents one or more characteristics of magnetic resonance signals from a small volume element or “voxel.” For example, the map can be stored as a three-dimensional array of data elements, the dimensions of the array corresponding to three-dimensional space. Data elements corresponding to a given plane in three-dimensional space can be selected for display in a two-dimensional picture such as a screen display or printed image. Each small area element on the surface of the picture, commonly referred to as a “pixel,” is assigned an intensity or color value based on the numerical values of the data element for the corresponding voxel.
MRI has been widely adopted in the medical arts. Because MRI does not use X-rays or other ionizing radiation, it offers safety advantages over techniques such as conventional X-ray imaging, fluoroscopy and CAT imaging.
Moreover, MRI allows visualization of tissues which are difficult or impossible to depict using other techniques. Magnetic resonance imaging can show abnormal tissues in contrast to surrounding normal tissues. For example, as disclosed in U.S. Pat. No. 3,789,832 of Raymond V. Damadian, magnetic resonance signals from malignant tumors have a characteristic referred to as the spin-lattice relaxation time or “T1” different from the T1 of normal tissues. If a magnetic resonance image is taken so that the data in each data element depends at least in part on the T1 of the tissue at the corresponding location, a picture showing malignant tumor tissue in contrast to normal tissue can be displayed.
MRI is also particularly useful in imaging the spine. MRI can depict the vertebrae in conjunction with related tissues such as the lamina or “discs,” as well as nerves, muscles and other neighboring tissues.
In magnetic resonance angiography, the magnetic field gradients applied during imaging, and the characteristics of the magnetic resonance signals which are translated into the image, are selected according to principles well-known in the art so that the data in voxels within arteries differs from the data for voxels in other structures, so that the arteries can be depicted in contrasting color or density to surrounding tissues. For example, arterial blood has a significant velocity and the surrounding tissues are nearly stationary. A so-called “motion-sensitive” MRI technique can be used so that a characteristic of the magnetic resonance signals from each voxel depends on the velocity of matter within the voxel. Magnetic resonance angiography yields images directly analogous to those obtained by conventional angiography, without the need for X-ray exposure. In some cases, MRI angiography can be performed without injection of a contrast medium. Moreover, MRI angiography can provide three-dimensional imaging information, so that images from any desired perspective can be displayed.
However, magnetic resonance imaging procedures have suffered from significant limitations. Conventional MRI equipment requires that the patient lie in a supine position on a horizontal bed which fits with the patient receiving space of the static field magnet. Some medical conditions have effects which change with posture. For example, a spinal disc may impinge on a nerve or other surrounding structure only when the patient is in an upright posture so that the disk is compressed by the patient's weight. Various proposals have been advanced to allow MRI procedures to be performed on patients in a posture other than the conventional supplying of posture. For example, Japanese published Patent Application No. 1-242056 published Sep. 27, 1989 depicts a magnetic resonance imaging unit with a tilting bed for supporting the patient in a supine position or in a standing position. Yoshida, U.S. Pat. No. 5,008,624 depicts a magnetic resonance imaging instrument with movable static field magnet in conjunction with a patient carrier which supports the patient in “various postures.” Palkovich et al., U.S. Pat. No. 5,779,637 discloses a system in which the patient lies supine within the static field magnet during one imaging procedure. The entire system, including the static field magnet and the patient can be pivoted so as to swing the magnet, the patient bed and the patient as a unit to a different position in which the patient bed extends vertically and the patient in an upright posture. A further image is taken in this position. None of these systems have been widely adopted.
Co-pending, commonly assigned U.S. patent application Ser. No. 09/718,946, filed Nov. 22, 2000 (“the '946 application”), now U.S. Pat. No. 6,677,753, the disclosure of which is hereby incorporated by reference herein, and co-pending commonly assigned U.S. patent application Ser. No. 09/789,460 (“the '460 application”), now U.S. Pat. No. 6,414,490, the disclosure of which is also incorporated by reference herein describe additional MRI magnet structures and patient handling devices as well as additional imaging methods. As disclosed for example in certain embodiments of the '946 application, a patient support such as a bed which can both tilt and elevate can be used in conjunction with a static field magnet to allow imaging of a patient in various orientations and to position various portions of the patient's anatomy in the appropriate location relative to the magnet for imaging. Discussion of the '946 and '460 applications in this background section of the present application should not be taken as an admission that the same constitute legally available prior art with respect to the present invention.
Most commonly, pictures derived from MRI images are read by a physician visually examining the picture to diagnose disease which may be present or to evaluate the progress of a known disease. Such evaluation may involve, for example, a mental comparison by the physician with pictures the physician has previously seen of normal and other diseased patients or pictures taken in the past of the same patient. This task requires careful examination and considerable professional judgment. Even with the capabilities achievable in MRI imaging, it is not always easy to spot disease states or changes in the patient's condition. Lemelson et al., U.S. Pat. No. 5,878,746 proposes an automated process in which the computer examines a new image to extract “features relating to particular disease states” using a pattern recognition technique and stores signals descriptive of these features in a “fact database.” These “feature signals” are compared with similar “feature signals” extracted from previously acquired images and the resulting comparison information is subjected to artificial intelligence rules to provide “a diagnostic assessment.” Perhaps because of the extraordinary difficulty of developing appropriate automated tools for finding features relating to disease states and rules for deriving diagnostic assessments from the compared features, this approach has not been adopted widely, if at all, in practice. Thus, the physician still generally faces the task of visually observing a picture of a patient derived from a particular MRI imaging session and mentally comparing that picture with either a prior picture of the same patient or a mental image of a “normal” anatomy. In this process, the physician typically attempts to discern the outlines of body structures in the picture.
As disclosed in Apicella et al., U.S. Pat. No. 5,273,040, the volume of blood contained within the ventricles of the heart can be determined from an MRI image. To do this, the physician, or an automated system must accurately identify the boundary of the blood-containing ventricle. To enhance the accuracy of an automated process for detecting where the boundary lies, two MRI images taken through the patient's heart in rapid succession, as, for example, during successive heartbeats, are mathematically superimposed and subtracted from one another to yield a “difference image.” Because the two MRI images are taken at two slightly different points in the cardiac cycle, the ventricles will be of slightly different sizes in the two images. Subtraction of the images will eliminate essentially all of the data, leaving only a small border or line having a width corresponding to the change in size between the first and second images. The difference image thus provides a clear line indicating the border of the ventricle which can be recognized in an automated system.
As disclosed in Bani-Hashemi et al., U.S. Pat. No. 5,647,360, digital subtraction angiography or “DSA” typically is performed using a computerized tomography or “CT” x-ray scan. A first set of CT scan data is obtained before injection of a contrast agent into the blood vessels, whereas a second scan is obtained a few minutes later, after injection of the contrast agent. The two data sets are registered with one another and subtracted from one another to yield a difference image which shows the blood vessels in high contrast. The registration procedure used to correlate data elements at corresponding locations within the patient in the two images with one another for subtraction operates by applying a mathematical transformation to one or another of the data sets. Different transformations are used for different parts of the transformed image so as to compensate for warping or non-uniform motion of different parts of the patient. The transformation for each portion of the image is determined using a pattern recognition procedure to match corresponding features shown in each portion of the image with corresponding features shown in the other image. The Bani-Hashemi patent suggests briefly that the registration technique “can be useful in various imaging systems, such as CT, MRI, PET, etc.”
Despite all of the effort devoted in the art heretofore to development of imaging systems and techniques, still further enhancements would be desirable.